The invention relates to a method and device for diagnosing deviations in a single cylinder lambda control in an internal combustion engine having at least two cylinders and an exhaust gas sensor designed as a broadband lambda sensor, wherein a pump current is evaluated by means of a pump cell and said pump current is used at least temporarily for an individual cylinder lambda control.
A lambda control in combination with a catalytic converter is today the most effective emission control method for the Otto engine. The use of a three-way or selective catalytic converter is particularly effective. This kind of catalytic converter has the capacity to degrade hydrocarbons, carbon monoxide and nitrogen oxides up to more than 98% in the event that the engine is operated in a range of approximately 1% around the stoichiometric air-fuel ratio whereat λ=1. The lambda value thereby indicates how far the actual, present air-fuel mixture deviates from the value λ=1, which corresponds to a mass ratio of 14.7 kg air to 1 kg gasoline theoretically necessary for complete combustion, i.e. the lambda value is the quotient from the air mass supplied and the theoretically required amount of air. In the case of excess air, λ>1 (lean mixture). In the case of excess gasoline, λ<1 (rich mixture).
When a lambda control is being performed, the exhaust gas is measured and the fuel quantity supplied is immediately corrected in accordance with the measurement result by means of a fuel injection system.
Lambda probes are used as detecting elements, which can be designed on the one hand as a so-called two-point lambda probe or discrete-level sensor and on the other hand as a continuous lambda probe or broadband lambda probe. The effect of these lambda probes is based in a manner known per se on the principle of a galvanic oxygen concentration cell with a solid state electrolyte. The characteristic curve of a two-point lambda probe has a sharp drop in the probe voltage at λ=1. For that reason, a two-point lambda probe, which is usually mounted directly behind the exhaust manifold, essentially allows only for the distinction between rich and lean exhaust gas. On the other hand, a broadband lambda probe permits the exact measurement of the lambda value in the exhaust gas over a wide range around λ=1. Both types of lambda probe consist of a ceramic sensor element, a protective tube as well as cables, a plug and the connections between these elements. The protective tube consists of one or a plurality of metal cylinders having openings. Exhaust gas enters through said openings by means of diffusion or convection and travels to the sensor element. The sensor elements of the two types of lambda probes vary thereby in the construction thereof.
The sensor element of a two-point lambda probe consists of an oxygen ion-conductive electrolyte, in the interior of which a cavity filled with a reference gas is situated. The reference gas comprises a certain constant oxygen concentration but otherwise no oxidizing or reducing constituents. In many cases, the reference gas is air. Electrodes, which are connected to plug contacts via cables, are mounted on the outside of the electrolyte which is in contact with the exhaust gas as well as on the inside of the cavity. According to the Nernst principle, an electrical voltage occurs across the electrolyte, denoted below as Nernst voltage which is determined by the concentration of oxidizing and reducing exhaust gas components in the exhaust gas and in the reference gas. If besides oxygen there are no oxidizing or reducing exhaust gas components in the exhaust gas, the Nernst voltage is described by the equationUNernst=URef−UAbgas=(R*T/4*F)*In(p02,Ref/p02,Abgas)
In this equation, URef stands for the electrical potential on the reference gas side, UAbgas for the potential on the exhaust gas side, p02,Ref and p02,Abgas for the oxygen partial pressure in the reference gas or respectively the exhaust gas, T for temperature, R for the general gas constant and F for the Faraday constant. The Nernst voltage can be tapped via the plug contacts and represents the signal of the two-point lambda probe.
The sensor element of a broadband lambda probe has an aperture on the surface, through which exhaust gas enters. A porous layer adjoins the inlet aperture, said exhaust gas diffusing through said porous layer into a cavity. Said cavity is separated from the external exhaust gas by an oxygen-ion conductive electrolyte material. Electrodes, which are connected to plug contacts via cables, are situated on the outside of the electrolyte as well as on the side of the cavity. The electrolyte situated between them is denoted as a pump cell. In addition, a reference gas having a certain constant oxygen concentration is situated in the interior of the sensor element, separated from the cavity by the same electrolyte material. An additional electrode, which is also connected to a plug contact, is situated in contact with the reference gas. The electrolyte between said additional electrode and the cavity side electrode is denoted as the measurement cell.
According to the Nernst principle, an electric voltage is applied across the measurement cell, which is referred to below as measurement voltage and is determined by the concentration of oxidizing and reducing exhaust gas components in the cavity and in the reference gas. Because the concentration in the reference gas is known and invariable, the dependence on the concentration in the cavity is reduced.
In order to operate the lambda probe, said probe must be connected via the plug to an evaluation unit, which, e.g., is situated in an engine control device. The measurement voltage is detected by the electrodes and transmitted to the evaluation unit. A control circuit is located in the control unit, said control circuit maintaining the voltage across the measurement cell to a set point value by a so-called pump current being driven through the pump cell. Because the current flow in the electrolyte takes place by means of oxygen ions, the oxygen concentration in the cavity is influenced. In order to maintain the measurement voltage at a constant level during steady-state operation, exactly as much oxygen has to be pumped out of the cavity during operation with a lean air-fuel ratio (λ>1) as diffuses through the diffusion barrier. On the other hand, during operation with a rich air-fuel ratio (λ<1) so much oxygen has to be pumped into the cavity that the diffusing, reducing exhaust gas molecules are compensated. While taking into account the fact that the oxygen balance in the cavity is maintained at a constant level by the pump current controller, a linear connection between the diffusion current, and thereby the pump current, and the oxygen concentration in the exhaust gas results from the diffusion equation. The pump current is now measured in the evaluation unit and transmitted to the main computer of the engine control device. It follows from that which is stated above that the pump current represents a linear signal for the oxygen balance in the exhaust gas. The connection between the lambda value and the oxygen balance is in fact non-linear, as the following equation proves.i)C02,Abgas=(1−1/λ)C02,Air  (2)
The curvature of the curve is however sufficiently small in the region which is relevant for the engine control in order to permit an exact determination of the lambda value from the pump current.
Broadband lambda probes are, for example, known from the German patent publication DE 10 2005 061890 A1 as well as from the German patent publication DE 10 2005 043414 A1, wherein the publication DE 10 2005 061890 A1 describes the design of a broadband lambda probe, in which provision is made according to the invention for the use of certain chemical elements during the construction thereof.
In internal combustion engines comprising two or more cylinders, which discharge the exhaust gas into a exhaust manifold, the pipes of which open into a common exhaust pipe, the lambda values of the individual cylinders can vary either due to different air charges caused, for example, by pressure surges in the intake manifold or due to different fuel quantities caused, for example, by tolerances of the injection valve or due to a combination of both causes. Such individual cylinder lambda fluctuations can adversely affect the performance of the engine as described below.
If, for example, a three-way catalytic converter is installed in the exhaust gas pipe and the exhaust gas from the individual cylinders is unevenly distributed across the cross section of the catalytic converter, a satisfactory conversion of the exhaust gas is not possible. In a catalyst segment which is exposed to lean exhaust gas, the oxidizing exhaust gas components cannot be converted; whereas in a catalyst segment which is exposed to a rich exhaust gas, the reducing exhaust gas components cannot be converted. In addition, the efficiency decreases and the fuel consumption thereby increases if a complete combustion of the fuel does not take place in a cylinder operated with a rich air-fuel ratio. Furthermore, incompletely combusted fuel from the cylinders operated with a rich air-fuel ratio and excess air from the cylinders operated with a lean air-fuel mixture can after-react in the exhaust pipe. Energy is thereby released which can lead to a thermal overstressing of and even to damage to the components installed in the exhaust gas system, in particular the catalytic converter.
It is therefore desirable in a closed control circuit to not only adjust the mean lambda value of all the cylinders to a set point value but also said mean lambda value of each individual cylinder. Such a method is denoted below as an individual cylinder lambda control. In addition, the American on-board diagnostics regulations (OBD) for the model year 2011 require a detection of individual cylinder lambda fluctuations, which is also referred to below as out-of-tune diagnostics or fuel trim diagnostics.
Single cylinder lambda controls are already known from prior art. Thus, the German patent publication DE 102 60 721 A1, for example, describes a method and a device for diagnosing the dynamic properties of a lambda probe, which is used at least temporarily for an individual cylinder lambda control. The method is thereby characterized in that at least one manipulated variable of the lambda control is measured and compared with a predefinable maximum threshold. In the event of the maximum threshold being exceeded, the dynamic behavior of the lambda probe is evaluated as being insufficient with regard to usability for the individual cylinder lambda control.
Prior art or respectively the subject matter of earlier patent applications uses the lambda signal of a two point lambda probe or a broadband lambda probe for an out-of-tune diagnostics or an individual cylinder lambda control. In so doing, a number of difficulties arise.
One difficulty is that the relevant frequencies of the lambda signal are damped. A significant damping is caused by the protective tube. This problem relates to both two point as well as broadband lambda probes. In the case of a broadband lambda probe, still further damping effects can in fact be added, namely as a result of the diffusion barrier and as a result of the pump current regulator depending on the design thereof. All of the damping effects act in a cumulative way. Frequencies in the actual lambda value created by individual cylinder fluctuations can be damped in a speed range around 2000 rpm by over 50% by means of the diffusion barrier. At higher rotational speeds, the damping continues to increase. The signal-to-noise ratio worsens which impairs the out-of-tune diagnosis as well as the individual cylinder lambda control. Viewed in terms of damping, a two point lambda probe can therefore have advantages with respect to a broadband lambda probe in the range around λ=1.
A broadband lambda probe has however also advantages with respect to a two point lambda probe. One advantage is that a lambda control with a broadband lambda probe can constantly adjust the mean lambda to a set point value. In contrast, the typical method used with a two point lambda probe, the so-called two point control, causes an oscillation in the lambda probe signal and thus adjusts only the mean value over time to the set point value. The individual cylinder lambda fluctuations are superimposed by the much stronger oscillations resulting from the control intervention such that the detection is impaired.
In addition, a method is known, in which an observer algorithm for the individual cylinder lambda values is supported by the measured value of a broadband lambda probe. Because the observer algorithm is based on the model of the system, which has the individual cylinder lambda values as input variables and the lambda mean value as output variable, said algorithm will be referred to below as the model supported method. An important parameter for the observer algorithm is the operating point dependent dead time of the lambda probe. The method is thereby impaired in that the dead time varies with production bandwidth and ageing. In order to resolve this difficulty, a dead time adaption method is described, which is however likewise afflicted with disadvantages. An active fuel adjustment is thereby required for the adaption. In addition, said adaption can only insufficiently depict a possible operating point dependency of the dead time variation.